The present invention relates to a selective ringing receiving device and method used for bit synchronization in a direct conversion system.
Conventionally, in such a kind of selective ringing receiving device, with respect to the digital signal received by a double superheterodyne system or single superheterodyne system, its changing point for bit synchronization was uniquely set for a rising changing point, a falling changing point or both rising changing points.
The conventional selective ringing receiving device, however, has the following defect. The changing point for bit synchronization is uniquely set so that the ringing receiving device in a direct conversion system cannot make correct bit synchronization if the outputting frequency in a receiving station or the transmission frequency in a base station varies.
Such a defect is due to the following signal receiving format in the direct conversion system in the case where there is no fluctuation in the outputting frequency in a receiving station and the transmission frequency in a base station: ##EQU1##
In the above receiving format, if a transmission speed is 1200 bps, a frequency shift is 4 KHz, and a frequency fluctuation .DELTA.f.sub.01 is 1 KHz, the resolution for demodulation a `1010` signal will be 3 KHz (.DELTA.f-.DELTA.f.sub.01) or 5 KHz (.DELTA.f+.DELTA.f.sub.01).
If the changing point is detected at the fixed point of its rising and falling, the resolution of the changing point is fixed to either one of enhancement and attenuation in engagement with the resolution in demodulation. Therefore, if a frequency fluctuation occurs in the attenuation direction, the changing point detection circuit does not operate normally, thus making it unable to assure the bit synchronization.